1. Field of the Invention
The invention relates to a cable roller according to the preamble of claim 1.
2. Description of the Related Art
On building sites, for compacting concrete internal vibrators are often used, the asynchronous motors of which have to be supplied with a special voltage and a special frequency. If, for example, an asynchronous motor provided in the internal vibrator and having a connected flyweight is to reach a rotational speed of 12,000 rpm, then in the case of a double-pole machine an excitation frequency of 200 Hz is required. From the public electricity supply, however, only a frequency of 50 Hz is available.
This is why in the past frequency converters were used, which were mostly supplied with three-phase alternating current and owing to their heavy weight were very difficult to handle. Soon, by virtue of the provision of corresponding power electronics with suitable semiconductors it became possible to miniaturize the frequency converter and thereby reduce its weight from e.g. 60 kg to 4 kg. Such a lightweight frequency converter was however still connected between the terminal and a cable drum, which was necessary for bridging larger distances. Moving the terminal, e.g. the internal vibrator, to various work sites therefore also entailed moving the frequency converter and, in addition, the cable drum.
From DE 197 22 107 C1 a cable roller is known, in which a frequency converter is integrated, with the result that the cable roller and the frequency converter may be moved as a unit.
FIG. 1 shows such an arrangement with a power cable 23, which is wound on a drum 22, wherein the drum 22 is carried by a rack 25. Integrated in the drum 22 is a frequency converter 24, which supplies at least one socket-outlet 21 with a special electric frequency. Connected to the socket-outlet 21 is a known internal vibrator 20 with an asynchronous motor. Such a combination of cable drum and frequency converter however has the drawback that during take-up or pay-out of the cable the socket-outlets 21 co-rotate with the drum. Consequently, the feeder cable for the internal vibrator 20 may become twisted.